Water heater storage tanks are used for storing and supplying hot water to residential and commercial properties. A typical residential water heater holds about fifty gallons (190 liters) of water inside a steel reservoir tank. A thermostat is used to control the temperature of the water inside the tank. Many water heaters permit a consumer to set the thermostat to a temperature between 90 and 150 degrees Fahrenheit (F) (32 to 65 degrees Celsius (C)). To prevent scalding and to save energy, consumers may set the thermostat to heat the reservoir water to a temperature in a range between 120.0 degrees F. to 140.0 degrees F. (about forty-nine degrees C. to sixty degrees C.).
A water heater typically delivers hot water according to the thermostat temperature setting. As a consumer draws water from the water heater, the water temperature in the water heater usually drops due to cooler supply water displacing the heated water in the storage tank. As the thermostat senses that the temperature of the water inside the tank drops below thermostat's set point, power is sent to the electric resistance heating element (or a burner in a gas water heater). The electric elements then draw energy to heat the water inside the tank to a preset temperature level.
Water heating may constitute 10-15% of household energy usage, totaling 7 to 14 kWh per day. In some locations of the United States and globally, the cost for electrical energy to heat water can depend upon the time of day, day of the week and season of the year. In areas of the United States where energy is at a premium, utility companies often divide their time of use rates into off-peak and on-peak energy demand periods with a significant rate difference between the periods. For example, energy used during off-peak hours may cost the consumer in United States dollars around 5 cents to 6 cents per kilowatt hour (kWh), while on-peak period energy may cost anywhere from 20 cents per kWh to $1.20 or more per kWh.
Household energy demands typically correspond to on-peak energy periods where the cost to produce the energy may be at a maximum for the utility company and the cost to use the energy may be at a maximum for the customer. Various conventional energy saving techniques have been utilized in an attempt to minimize the cost of energy to both the utility company and the consumer.
One approach may be to use a programmable timer to turn off the entire water heater or the lower element. For example, a clock timer could be used to provide planned heating periods during known off peak periods of the day. While this approach is possible, adapting to period variation in the rate schedule and emergency load shedding request signals from the utility are not accommodated.
Another approach is to increase the storage size of the tank and/or increasing the set temperature of the tank in combination with use of a thermostatic mixing valve at the hot water outlet. Hot water capacity may be increased, but it does not alter the energy consumption pattern of the water heating system. A lower heating element may also need to be disengaged in order to avoid consumption during “on peak” energy rate hot water usage.
Set point alteration is another means to reduce heating events during on peak water usage. While this may produce a similar outcome as disengagement of the heating elements, it requires a substantially different control mechanism for regulation and limiting of the tank temperature and cannot be easily retrofitted to an existing water heating system.
Alternatively, the entire water heater may be shut off during on peak energy periods. This could result in the consumer running out of hot water during peak hours and left to wait until off peak hours to resume heating the entire stored water volume of the tank, meeting demand. This approach requires consumer behavior change or purchase and installation of a larger storage tank size to bridge the peak hour water usage. This results in an investment requirement from the consumer and presumes the availability of space to install a larger tank. Commonly, space limitations and/or standardization of heater sizes may prevent installation of a water heater large enough to meet the storage needed to bridge the peak hours.
Accordingly, a need exists for providing an energy storage method and apparatus that allows for storage during low demand energy production times. In addition, it would be advantageous to have communications between the utility provider and the energy storage system in order to improve overall efficiency.